Cellular and viral RNA sequences homologous to type C and type B tumor viruses have been studied. These sequences have been characterized in terms of their rescuability by heterologous RNA tumor viruses. The distribution and biochemical behavior of rat cellular RNA homologous to two murine sarcoma viruses has also been studied. The inducibility of type B murine mammary tumor virus by dexamethasone in interspecies somatic cell hybrids has been shown to be cis-dominant. Finally, two additional type C viruses have been isolated from a lymphomatous P. hamadryas baboon.